No you don't need a gambling license as long as you stay under the radar. As long as you don't advertise your game on newspapers, TV or places where regular businesses advertise.Your top priority should be making your game addictive. That will create revenue in itself without the need to promise cash payouts.What you should ask yourself, and that is more of a design issue, is whether using real money will make your game more addictive. I can say almost certainly probably not, because your economy is not to scale.And can only create you legal issues.We live in the era of gaming/gambling, where the line between the two is vanishing. Most countries are only now creating laws to tackle this.I have lost track of how many established games are just disguised gambling.I will not name them here because I don't want to get into trouble myself.They are even worse than illegal casinos, because these games are aimed at kids.Nobody will care what you do until you become a big fish.